1. Field of the Invention
The present invention relates to a method for making spinnable balls, a method for making a display medium and a method for making a display device.
2. Description of the Related Art
In recent years, the need for thin display devices with low electricity consumption has been increasing with the development of information devices, encouraging intensive research and development of display devices to satisfy this need. In particular, there has been intensive development of liquid crystal devices which satisfy this need, since electrical control of the arrangement of liquid crystal molecules causes a change in optical characteristics. In current liquid crystal devices, however, character visibility on a display deteriorates when the angle of vision or reflected light is inadequate. Further, flicker or low luminescence of the light source strains the eye. Energetic study of novel display devices which reduce eyestrain is now in progress.
N. K. Sheridon et al. disclosed a new type of display device that uses the twisting of fine balls driven by an electric field ("A Twisting Ball Display", Proc. of the SID, Vol. 18, No. 3/4, p. 289, 1977; U.S. Pat. Nos. 4,126,854, 4,143,103 and 5,389,945; and Japanese Patent Laid-Open No. 64-42683). The display device uses fine balls each consisting of a white hemisphere and a black hemisphere. The fine balls are placed in cavities formed on a carrier. The cavities are filled with a highly resistant liquid so that balls can rotate without restriction. Charging states of the black and white hemispheres differ from each other in a certain type of liquid, and application of an external electric field can control the spinning of the fine ball such that a white or black hemisphere faces the observation side. Display of an image is achieved in such a manner. Such a mechanical type of display system is extremely stable against temperature change and noises from electrical disturbance. Since the display can be memorized, the unchanged display does not consume electricity. Further, the display using reflection and scattering of natural light on the ball surfaces is an ideal display that can reduce eyestrain caused by flicker of the light source which is inevitable in CRT displays.
Known methods for making dichroic balls used in such a display device include a method for joining and solidifying two melted droplets having different colors in a host liquid, and a method for joining and solidifying two melted droplets having different colors by centrifugal force (Japanese Patent Publication No. 7-67754 and Japanese Patent Laid-Open No. 5-279486). The former method forms dichroic balls by solidification of a melted material in the host liquid, whereas the latter method forms dichroic balls by centrifugal solidification of a melted material.
These conventional methods, however, have the following problems. Since these balls are generally made of organic thermoplastic resins, selection of usable materials is limited. Also, selection of a usable liquid is limited, since some types of liquid will swell rotating organic colored balls.
M. Saitoh et al. disclosed another method for making colored balls used in the above-described display device in Proc. of the SID, Vol. 23/4, pp. 249-252, 1982. According to this method, fine white balls are embedded onto an adhesive layer provided on a substrate, and a black layer is formed on the exposed hemisphere of each white ball by a vacuum deposition process. M. Saitoh et al. does not mention the problem that the particle sizes of the produced balls are not equalized. Further, the black layers formed on the white hemispheres of the fine balls are not equalized.